Buried defects induced by plasma assisted molecular beam epitaxy of AlN and GaN on Silicon

The present work is dedicated to the study of peculiar defects observed in GaN/AlN structures grown on Silicon substrate using the plasma assisted Molecular Beam Epitaxy under metal-rich conditions. Optical microscopy shows that these defects have undefined shape when AlN is grown on Silicon. On the contrary such defects are not observed with AlN films grown under nitrogen-rich plasma assisted or ammonia source Molecular Beam Epitaxy, but triangular shape defects appear in case of GaN regrowth on the latter. As scanning electron microscopy is not able to evidence these defects but only protuberances connected with them, we deduce that they are located in the Silicon substrate. Complementary analysis with atomic force microscopy and energy dispersive X-ray analysis lead us to propose a scenario involving the etching of the Silicon substrate with Ga or Al diffusing through weak points in the nucleation layer.